: Fetal sheep will be exposed to intraamniotic endotoxin and the minimal effective dose determined as well as the effects of prolonged low dose exposure. Subsequently, the route of the effects of endotoxin will be evaluated by targeting the stimulus to the amniotic cavity, the systemic circulation, the GI tract, or the lungs. All lambs exposed in utero will be delivered preterm and evaluated for indicators of amniotic space inflammation, lung function, and lung biochemical indicators of maturation. Candidate mediators, identified by evaluating the fetal responses to endotoxin, will be tested for their ability to induce fetal responses and lung maturation.